2 Point Measurement

I've been working a simple calculator for a kind of RPG I'n developing, and it requires use of a grid.

I have a set number of checkboxes in a grid formation, where the user checks off two of these boxes.

From here, I'm trying to get the measurement between the two checked boxes... my friend says there is a function that can measure the total pixels between two points, but I wasn't quite certain how that would work.
In fact, I was hoping for more of a coordinate system, where the distance could be measured horizontally and vertically in terms of the number of boxes transversed. Each checkbox would have an X and Y value, therefore. Then, I'd just use the Pythagorean theorem to determine the straight distance between these selected boxes.

I don't know how simple this could be, I just started to try out Visual Basic again.

If anyone has an idea, I'd really appreciate any help. Thanks in advance.
This is on Visual Basic 2008, for reference.